Coordinate-measuring machines (CMM) of various construction are known for the tactile measurement of objects. All of these machines have a measurement arm which can be positioned in three directions in space, and the end of the measurement arm carries a probe head with a probe which mechanically contacts the object or workpiece to be measured.
These machines are poorly suited for uses in which very large workpieces must be measured or in which a great density of points is required, since for each measurement point the probe must be moved into contact with the workpiece, resulting in relatively long measurement times.
It has, therefore, also already been proposed to measure such objects optically by three-dimensional triangulating intersection from at least two points, as for example, by means of two theodolites.
However, no significant reduction in measurement time can be obtained with simple manually operated optical systems of this kind, since each measurement point must also be targeted directly by the operator, using both optical sighting devices.
It has, therefore, already been proposed to motorize the axes of the theodolites and to include electronic sensors in the form of CCD cameras in the ray path of the observation telescope of each theodolite; the CCD cameras produce images which are then automatically evaluated in an associated image-processing unit. One such measurement system is described in VDI Berichten No. 711, 1988, pages 139-151.
However, the known system is very cumbersome and expensive since it contains a large number of components that are unnecessary in a system for measuring industrial objects, since the theodolites used are special instruments for geodesy and contain, for example, compensators, leveling devices, mechanical coarse/fine drives for angle-setting, and multiply scanned angle circles; these are among a number of components which are necessary solely for manual operation or for geodetic uses, but they are not needed for measurements on industrial objects.
On the other hand, theodolites supply angular values which are usable only in a leveled state whereby to assure a vertical axis that is truly vertical. This, in its turn, greatly limits the usefulness of a theodolite system for the measurement of industrial objects.
Substantially greater precision of measurement is definitely required for industrial projects than in geodesy, since the dimensions of measured objects are to be determined with a precision of a few .mu.m. The need for precision requires that, if possible, the relation in space between sensors used for measurement and the object itself being measured, shall not change during a measurement cycle and that this relation be retained in stable manner. With theodolites that are simply tripod-mounted alongside the object to be measured, this requirement can be satisfied only with great difficulty since the positional relationships between object and sensor change as a result of vibrations, temperature influences, etc.
EPO patent application A1-0 256 968 describes a method for non-contact measurement of large objects, using two video cameras which are mounted for selective rotation on each of two orthogonal axes. To determine coordinate values, the signals of these cameras are fed to a computer, along with orientation-angle data from transmitters associated with the respective axes of rotation. Aside from the fact that this EPO patent application does not disclose the construction of the two-axis rotary device, this EPO case does not have anything to say regarding the problem of fixing the positional relation between the measurement object and the sensor.
Finally, U.S. Pat. No. 4,226,536 describes a non-contact optically-operated measurement system, illustratively for measuring the contour of helicopter-rotor blades, in which two mark projectors and two electro-optical trackers are mounted on a common frame which is movable on rails so that optical intersects can be produced over the entire length of the object and measured by the triangulation principle.
This system also operates relatively slowly since electro-optical tracking of the projected light spot which is moved over the surface of the object must be accurate. Constant positional relationships between the measurement system, on the one hand, and the object to be measured, on the other hand, depend on the quality of the bearings and guides used for the displaceable frame and cannot be definitely maintained within the above-indicated measurement-precision range of a few .mu.m.
Furthermore, at least two picture sensors are always required in all the known measurement devices.